There are numerous situations in which it would be advantageous to measure the distance between markers on an optical fiber. For example, in remote temperature sensing, the distance between the markers varies with average temperature of the section of fiber between the markers. Hence, a measurement of the distance between the markers allows one to infer the average temperature of the environment in the vicinity of the fiber connecting the markers.
Similarly, the distance between the markers will change if the fiber is placed under a strain. This provides a means for measuring the deformation of a structure to which the optical fiber is attached. By incorporating fiber-optic sensor arrays into structures such as bridges, building frames, dams and tunnels, material strains can be monitored throughout the lifetime of the structure.
Low coherence interferometry is an attractive technique for this type of distance measurement. While low coherence interferometry maintains the accuracy of conventional single-frequency (i.e. long coherence length) interferometry, it avoids many of the limitations and problems associated with long coherence length signals. In particular, absolute length or time delay can be measured. This is in contrast to fixed-frequency coherent interferometry where only changes in delay can be measured. Also, due to the short coherence length of the sensing signal, undesirable time-varying interference from stray system reflections is eliminated. Another advantage of low coherence interferometry is the ability to coherence multiplex many sensors onto a single optical signal without requiring the use of relatively complex time or frequency multiplexing techniques.
Low coherence interferometry operates by detecting interference of a reference light signal with a light signal generated when light from the same source is reflected backwards down the fiber in which markers are placed. The two signals will coherently interfere when the optical path length of the reference path is the same as that of the reflected light signal. This interference is normally detected by varying the reference path length and observing the path length at which the interference at the output of a photodiode illuminated with light from both paths is maximized. The reference path length is often varied by moving a mirror that is part of the optical path.
While conventional low coherence interferometry can measure distances very accurately, the range of distances that can be measured is limited by the degree to which the mirror described above may be moved. In optical sensor arrays, sensor arrays consisting of a fiber that is many meters long with partially reflecting markers every meter are contemplated. Hence, a conventional low coherence interferometry systems would require a reference path length that must be varied over many meters. Such systems are not practical because a reference path having an optical path length that may be varied over many meters is difficult to construct.
Another problem with conventional low coherence interferometry is determining which marker is being measured. If a set of markers is located at equal spacing along a fiber, the output of a low coherence interferometry is a set of peaks that are separated by the distance between the various markers. Since the peaks are indistinguishable in this arrangement, there is no method for assigning a particular peak to a given marker unless the absolute location of at least one of the markers is known. In situations in which the fiber is built into a structure, the absolute distance to the first marker may not be known since there is often an unknown amount of fiber connecting the sensing array to the detector site. In principle, the location of the first marker may be determined by inserting various delays into a reference arm until a peak is detected; however, this process is time consuming, and hence, expensive.
Broadly, it is the object of the present invention to provide an improved apparatus and method for measuring the distances between markers on an optical fiber or the like.
It is a further object of the present invention to provide an apparatus that does not require a reference path that is equal to the distance to the most distant marker.
It is a still further object of the present invention to provide an apparatus and method that allows the signals generated by the various markers to be distinguished from one another at the detector.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.